1. Field of the Invention
The present invention relates to sublimation growth of silicon carbide single crystals with low dislocation density.
2. Description of Related Art
Silicon carbide is an important wide-bandgap material used for the development and manufacturing of SiC and GaN semiconductor devices of new generation. While GaN-based devices are intended for the operation at microwave frequencies, SiC-based devices are aimed at efficient power switching. Other applications are also envisioned and are emerging. Silicon carbide is used as a lattice-matched substrate material to grow epitaxial layers of SiC and GaN. In order to produce low-defect epilayers and high-quality devices, the substrate must have good crystal quality, including low dislocation density.
Dislocations are one-dimensional crystal defects. They are generally categorized based on the value and direction of the so-called Burgers vector, which represents the magnitude and direction of the lattice distortion of dislocation in a crystal lattice, and a direction of a dislocation line. In silicon carbide, dislocations with their lines extending along the crystallographic c-direction are called threading. These include threading screw dislocations (TSD) and threading edge dislocations (TED), which have their Burgers vector not exceeding 1 or 2 lattice parameters. Micropipes are hollow-core threading screw dislocations with a larger Burgers vector, reaching in some cases hundreds of lattice parameters. Dislocations with their lines parallel to the basal c-plane are called basal plane dislocations (BPD).
For the measurements of the dislocation density in SiC, etching in molten KOH is generally employed to reveal dislocation-related etch pits. Often, the nature of the particular dislocation can be determined based on the characteristic shape of the etch pit. A photograph of an etched SiC surface (c-plane) is shown in FIG. 1. In general, etch pits due to TSDs are hexagonal and somewhat larger than those due to TEDs, which often appear more rounded. Etch pits due to BPDs are usually elongated and asymmetrical.
As an example, in a typical 7.62 cm diameter PVT grown 4H—SiC substrate, the TED and TSD densities are on the order of 5·104 cm−2 and 104 cm−2, respectively, and the density of BPD can be 104 cm−2 or as high as 105 cm2. The total dislocation density can be as high as 3·105 cm−2. The measured dislocation density depends both on the quality of the crystal and on the way wafers extracted from the crystal are sliced. Etching can reveal dislocations only if their lines intercept with the surface of the wafer. Therefore, wafers that are sliced parallel to the c-plane (on-axis) tend to exhibit lower densities of BPDs.
Moreover, the shape of etch pits in heavily nitrogen-doped n+SiC substrates are usually not well defined. A fraction of them have clearly elongated shapes and can be designated as BPD, all the rest have near circular shapes.
There are usually no distinctly hexagonal shaped dislocation etch pits. It is possible that this might be related to a low occurrence of screw dislocations in such crystals. More likely, it results from the effect of nitrogen dopant on the etching behavior of SiC, whereupon the distinction between TSD and TED diminishes.
Moreover, these etch pits tend to be slightly asymmetric. This may indicate a basal plane component in these dislocations as suggested in “Basal Plane Dislocation Dynamics in Highly p-Type Doped Versus Highly n-Type Doped SiC”, by P. J. Wellman et al., International Conference on Silicon Carbide and Related Materials, 2005, Pittsburgh, or it may simply reflect the impurity induced non-uniformity during etching. As a result, it is difficult to clearly separate different types of dislocations through molten KOH etching methods on these kinds of samples. However, this difficulty does not affect the measurement of total dislocation density.
For 4H—SiC homoepitaxy, an 4H—SiC substrate is generally oriented off its c-plane by 4 or 8 degrees in order to achieve regular step flow and eliminate nucleation of the 3C polytype. Consequently, most of the threading defects and a fraction of BPDs penetrate from the substrate into the epilayer. A large percentage of BPDs convert into TEDs at the interface between the substrate and the epitaxial layer. In addition, new defects can emerge at the substrate-epitaxial layer interface as a result of interaction between various types of dislocations and the step flow. These epitaxial layer defects are detrimental to device performance and include so-called comets and triangles, as reported in “Structure of Carrot Defects in 4H—SiC Epilayers”, by X. Zhang et al., International Conference on Silicon Carbide and Related Materials, 2005, Pittsburgh; and “Surface Morphology of Silicon Carbide Epitaxial Films”, by J. A. Powell et al., J. Electronic Mat., 24, (1995) p. 295. Although the exact nature of these defects is not yet completely understood, it is believed they are closely related to the dislocations in the substrate.
FIGS. 2A and 2B are magnified photographs of epitaxial surfaces including a comet defect and a triangle defect, respectively. The photographs were taken on samples etched with molten KOH. Therefore, the surfaces also include a number of dislocation-related etch pits. As can be seen, both comet and triangle defects are bounded by dislocations. In particular, the comet is bounded by a TSD at one end and by a BPD at the other end. The triangle is bounded by two or more BPDs.
As is well-known to those skilled in the art of SiC growth and epitaxy, micropipes and dislocations have a detrimental impact on the efficiency and reliability of devices made with SiC, as reported in “Breakdown degradation associated with elementary screw dislocations in 4H—SiC p+n junction rectifiers” by P. D. Neudeck et al., Solid-State Electron. 42, (1998) p. 2157; and “Long term operation of 4.5 kV PiN and 2.5 kV JBS diodes”, by H. Lendenmann et al., Material Science Forum 353-356, (2001) p. 727. The reference “Bulk Crystal Growth, Epitaxy, and Defect Reduction of Silicon Carbide Materials for Microwave and Power Devices”, by J. J. Sumakeris et al., MRS Bulletin, Vol. 30, April (2005), p. 280 discusses device limitations due to the micropipes, TSDs, TEDs and BPDs that propagate from the substrate into the epi-layer.
In summary, reduction of dislocation density in the SiC substrate is crucial for minimizing the presence of harmful defects in epitaxial layers and for achieving improved characteristics of the devices.
The most common technique used for the growth of SiC single crystals by sublimation is Physical Vapor Transport (PVT). A schematic diagram of a PVT system 2 is shown in FIG. 3. Polycrystalline SiC grain, which serves as a sublimation source material 4, is placed at the bottom of a graphite growth crucible 6 and a SiC seed crystal 8 is attached to a graphite lid or top 10 of crucible 6. Loaded crucible 6 is heated to a growth temperature between 2000 and 2400° C. by way of a resistance heater (resistance heating) or an RF coil 12 (induction heating). A temperature difference is established between the sublimation source material 4 and seed crystal 8 in the presence of a low pressure (1-200 Torr) inert gas, such as helium or argon, inside crucible 6, whereby the temperature of source material 4 is higher than that of seed crystal 8. Under these conditions, source material 4 sublimes and fills the interior of the crucible 6 with a vapor comprised of silicon- and carbon-carrying volatile molecular species, such as SiC2, Si2C and Si. Driven by the temperature gradient, these species diffuse through the vapor phase in the direction from sublimation source material 4 to seed crystal 8. Due to the lower temperature of seed crystal 8, a supersaturation is created causing condensation of the vapor on seed crystal 8 and growth of a SiC single crystal 14 thereon.
The reference “Ultrahigh-quality silicon carbide single crystals,” by Nakamura et al., Nature 430 (2004), p. 1009, discloses growing SiC crystals with very low dislocation density, on the order of 100 cm−2, using repeated a-face growth (RAF). This procedure included multiple sublimation growth runs. At each run, the growth direction was turned by 90 degrees. Although dramatically reduced dislocation density was observed, the RAF process is not conducive to volume manufacturing and, thus far, no recognizable body in the SiC crystal growth field has independently repeated or confirmed the results.
The total population of dislocations in SiC crystals can be divided into two groups: dislocations inherited from the seed and those generated during growth. Generation of dislocations during growth is believed to be due to the contamination of the growth interface by particles and other contaminants released from the source and graphite parts. Steep temperature gradients can lead to the excessive thermo-elastic stress and generation of dislocations.
Conventional and well-established measures can be applied to the PVT sublimation growth process in order to eliminate or reduce the generation of dislocations during growth. These measures include better purity of the growth components; degassing and vacuum baking of the graphite furniture and source at elevated temperatures; and moderation of the temperature gradients.
The reference “Basal Plane Dislocation Dynamics in Highly p-Type Doped Versus Highly n-Type Doped SiC”, by P. J. Wellmann et al., International Conference on Silicon Carbide and Related Materials, 2005, Pittsburgh, reports the study of the incorporation of an n-type layer during the growth of aluminum doped p-type SiC, using over-compensation by nitrogen, and followed the fraction of BPD in total dislocations through the crystal. The focus of the study was on the comparison between p-type and n-type doped SiC and the effect on BPD. The reference reported the increase in BPD fraction in the inserted n-type layer, and subsequent decrease of the fraction. The effect on the overall level of dislocation density was not reported. Furthermore, as previously discussed, the difference in the observed etch pit morphology between p-type and n-type SiC does not necessarily correspond to the nature of dislocations. It is possible that the etching behavior is modified by different impurities. There is no known technique that could be applied to minimize the propagation of threading dislocations from the seed into the growing crystal.
It would, therefore, be desirable to reduce the dislocation density in the SiC single crystals grown by sublimation and, more specifically, to minimize the propagation of the threading dislocations during growth from the seed into the growing crystal.